Cryogenic Low Energy Astrophysics with Noble gases
The field of neutrino interactions is now the source of great excitement with the recent results from Super-Kamiokande, SNO, and KamLAND neutrino detectors having proven that neutrinos have mass. This is the first substantial change in the Standard Model of particle physics in the last 20 years. A second area of great excitement lies in the field of dark matter research. Observations of galaxy rotation curves clearly show that there is matter outside of the luminous region, suggesting the existence of dark matter. Measurements of the cosmic microwave background and gravitational lensing provide further evidence of the presence of dark matter in the universe. The exact nature of this dark matter remains elusive, but one well-motivated candidate is the Weakly Interacting Massive Particle or WIMP. As a result, there are a number of experiments that aim to directly detect the interaction of a WIMP with regular matter.
The requirements for a detector capable of observing neutrinos and dark matter are similar. We need a detector technology that can simultaneously provide low radioactive backgrounds, a low energy threshold, and large detector mass at reasonable cost. CLEAN is an idea that meets all these requirements using liquid neon or liquid argon as a detection medium.
The basic idea of CLEAN is to take advantage of the fact that both neon or argon scintillate when a exposed to radiation. When an energetic charged particle (such as the recoil from a neutrino-electron scattering event) passes through the liquid noble gas, it will produce scintillation light in the extreme ultraviolet range that can be detected using a combination of a wavelength shifting fluor and a photomultiplier tube. Because the energy of this light is less than the difference in energies between the ground state and first excited state of neon or argon, the liquid will be transparent to this light; therefore a detector based on a condensed noble gas can be built to essentially arbitrary size without signal loss from reabsorption.
Liquid neon has a number of advantages for detecting neutrinos or other weakly interacting particles. Because neon has no long-lived isotopes, it has no inherent radioactivity to create backgrounds in a sensitive detector. This separates neon from heavier noble gases which do contain radioactive isotopes and from organic scintillator materials which inevitably contains beta radiation from 14C. Neon also has very low binding energies to a variety of surfaces, allowing it to be effectively purified of radioactive contaminants using cryogenic traps. Another advantage is the its relative density compared to liquid helium; neon can be used efficiently as a self-shielding medium, while also allowing a smaller detector volume. These characteristics render neon a great choice as a neutrino detection medium.
A similar list of advantages apply to argon when it comes to detecting dark matter, with the added advantage that WIMPs have a much higher cross section for scattering with argon compared to neon due to the higher mass number. In addition, natural argon is extremely cheap. One major concern with argon is that it does have a radioactive isotope, 39Ar. However, a feature of scintillation in liquid noble gases is that the scintillation timing can be used to determine the type of excitation that occurred. Therefore, one can discriminate between a nuclear recoil that would characterize a WIMP-nucleon scattering event from an electronic recoil that would characterize the signal from 39Ar. This is called pulse shape discrimination or PSD; PSD is possible using neon, but it is much more effective in liquid argon.
Currently we are researching more about scintillation light in liquid neon and liquid argon. We have designed and built a 4 kg apparatus, and we have measured the light yield of argon to different excitations, as well as the PSD at low energies. We are repeating these measurements in liquid neon. During 2008, we will build a 100-kg liquid argon detector called Mini-CLEAN, which will be capable of setting a competitive dark matter limit.
To read more about CLEAN, follow this link or click on "Publications" in the main menu to find a list of further papers. Some pictures of our setups can be found here.
CLEAN Collaborators should go here. Another link of interest is Professor Orzel's page at Union College on using Atom Trap Trace Analysis to measure trace amounts of krypton in samples of other rare gases; we hope to use ATTA to monitor the few parts in 1015 Kr contamination needed for large CLEAN neutrino detector.